Eric's Weekly Post #3 (Warning, science enclosed. Do not fold or bend.)


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Message 524750 - Posted: 1 Mar 2007, 0:04:31 UTC - in response to Message 523553.

No worries, Clyde

What you got was pretty typical. Polaroid lenses are like the rest of us: imperfect. Therefore, although the theory says they will blank out the light, in practice, a little does get through. Obviously, as always, it depends on quality.

Radio waves are polarised by the orientation of the antenna. A standard whip antenna stands vertically, producing vertically polarised radio waves. The same whip placed horizontally will produce horizontal polarization. TV signals are almost universally horizontally polarised, by the way, which limits their range. Helical polarisation is achieved by a coiled antenna, which can be clockwise or anti-clockwise. Many "outside broadcast" units have Line-of-Sight (LOS) antenna, composed of coiled antenna enclosed in a plastic or glass fibre tube. You may have seen a news van or similar, prowling round sporting one on it's roof, often on a telescopic mast.

Actually, we (the larger "we" -- not any specific profession) tend to like vertical antennas because they radiate evenly in all directions. If you are running a broadcast station and you want to talk to people all around you, this is easiest.

Most sources of RF noise are also vertically polarized.

This is why you see TV antennas that are horizontally polarized -- both at the transmitting site, and at the receiving site. You get less noise this way.

Usually.
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Message 524752 - Posted: 1 Mar 2007, 0:05:30 UTC - in response to Message 524650.

The antenna, then, at the focus, is only one, 1/2 or 1/4 wavelength long?

While that isn't 100% true, it's a pretty fair generalization.
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Message 524924 - Posted: 1 Mar 2007, 10:55:42 UTC - in response to Message 524752.
Last modified: 1 Mar 2007, 10:58:15 UTC

The antenna, then, at the focus, is only one, 1/2 or 1/4 wavelength long?

While that isn't 100% true, it's a pretty fair generalization.


Dipoles can be much shorter than 1/2 wavelength. Theoretically, a half-wavelength dipole is ideal, but practically, this turns out to be about 5/8 wavelength. I believe this avoids problems with "standing waves". A dipole of one wavelength just doesn't work. Since there is a sine wave involved, half the antenna would be electrically of the opposite potential to the other half. They would cancel each other out.

If a dipole is physically less than half-wavelength, it is usually "fooled" into behaving as a half-wavelength by the use of a "loading coil". Basically, this is a rheostat (variable resistor) which can be adjusted. When CB users adjust the "SWR" (Standing Wave Ratio) of their antenna, this is what they are doing.

Ned's right... horizontally-polarised radio suffers less from noise, but TV stations not only transmit on the horizontal for that purpose. They also assign specific areas for their broadcast, and directional, horizontal antennae limit the broadcast area. Directional antennae also deliver more power to the desired broadcast area. Having said that, the British Services TV in Germany used vertical polarisation (probably still do) to prevent the civilian population from watching British TV!!!!!
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Message 525012 - Posted: 1 Mar 2007, 17:47:20 UTC - in response to Message 524924.



[snip]

If a dipole is physically less than half-wavelength, it is usually "fooled" into behaving as a half-wavelength by the use of a "loading coil". Basically, this is a rheostat (variable resistor) which can be adjusted. When CB users adjust the "SWR" (Standing Wave Ratio) of their antenna, this is what they are doing.

[snip]


Generally, the loading coil is a COIL, I.E an inductor, (sometimes variable) that makes the antenna look resonant to the transmitter at a specific frequency. (lowering the SWR) For CB, this would be in the 27.200 Mhz range. (CB channel 19, the "trucker's channel" is 27.185 Mhz, but the CB band is 27.005 to 27.405.) The CB antennas available commercially are usually long, though - you CUT them to the proper length for mid band. (called "tuning the antenna")
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Message 525346 - Posted: 2 Mar 2007, 12:26:51 UTC - in response to Message 525012.
Last modified: 2 Mar 2007, 12:30:09 UTC



[snip]

If a dipole is physically less than half-wavelength, it is usually "fooled" into behaving as a half-wavelength by the use of a "loading coil". Basically, this is a rheostat (variable resistor) which can be adjusted. When CB users adjust the "SWR" (Standing Wave Ratio) of their antenna, this is what they are doing.

[snip]


Generally, the loading coil is a COIL, I.E an inductor, (sometimes variable) that makes the antenna look resonant to the transmitter at a specific frequency. (lowering the SWR) For CB, this would be in the 27.200 Mhz range. (CB channel 19, the "trucker's channel" is 27.185 Mhz, but the CB band is 27.005 to 27.405.) The CB antennas available commercially are usually long, though - you CUT them to the proper length for mid band. (called "tuning the antenna")


All correct! :)
Mind you, I thought the practice of cutting the antennae to suit the transmitter had died out long ago. I suppose the added expense of a "SWR" meter puts it out of range for some people, and often the added performance just isn't needed.

[edit] The ideal length for a CB antenna with NO loading coil would be around 6.9 metres, hence most CB antenna have a built-in coil, usually at the base, but often in the centre of the "whip"
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Message 525620 - Posted: 3 Mar 2007, 3:29:20 UTC - in response to Message 525346.



[snip]

If a dipole is physically less than half-wavelength, it is usually "fooled" into behaving as a half-wavelength by the use of a "loading coil". Basically, this is a rheostat (variable resistor) which can be adjusted. When CB users adjust the "SWR" (Standing Wave Ratio) of their antenna, this is what they are doing.

[snip]


Generally, the loading coil is a COIL, I.E an inductor, (sometimes variable) that makes the antenna look resonant to the transmitter at a specific frequency. (lowering the SWR) For CB, this would be in the 27.200 Mhz range. (CB channel 19, the "trucker's channel" is 27.185 Mhz, but the CB band is 27.005 to 27.405.) The CB antennas available commercially are usually long, though - you CUT them to the proper length for mid band. (called "tuning the antenna")


All correct! :)
Mind you, I thought the practice of cutting the antennae to suit the transmitter had died out long ago. I suppose the added expense of a "SWR" meter puts it out of range for some people, and often the added performance just isn't needed.

[edit] The ideal length for a CB antenna with NO loading coil would be around 6.9 metres, hence most CB antenna have a built-in coil, usually at the base, but often in the centre of the "whip"

Well for a full wave that would be 11 meters. (it's in the 11 meter band!) But I've never known an antenna needing to be cut. Most have a screw at the base you can loosen to adjust the antenna length up or down. The biggest mobile you'll see is a 1/4 wave, or 8 feet. I had a 3/4 wave base Sigma 4. And for a while I had 2 of them cophased into a beam. ;)
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Message 525785 - Posted: 3 Mar 2007, 13:04:45 UTC - in response to Message 525620.



[snip]

If a dipole is physically less than half-wavelength, it is usually "fooled" into behaving as a half-wavelength by the use of a "loading coil". Basically, this is a rheostat (variable resistor) which can be adjusted. When CB users adjust the "SWR" (Standing Wave Ratio) of their antenna, this is what they are doing.

[snip]


Generally, the loading coil is a COIL, I.E an inductor, (sometimes variable) that makes the antenna look resonant to the transmitter at a specific frequency. (lowering the SWR) For CB, this would be in the 27.200 Mhz range. (CB channel 19, the "trucker's channel" is 27.185 Mhz, but the CB band is 27.005 to 27.405.) The CB antennas available commercially are usually long, though - you CUT them to the proper length for mid band. (called "tuning the antenna")


All correct! :)
Mind you, I thought the practice of cutting the antennae to suit the transmitter had died out long ago. I suppose the added expense of a "SWR" meter puts it out of range for some people, and often the added performance just isn't needed.

[edit] The ideal length for a CB antenna with NO loading coil would be around 6.9 metres, hence most CB antenna have a built-in coil, usually at the base, but often in the centre of the "whip"

Well for a full wave that would be 11 meters. (it's in the 11 meter band!) But I've never known an antenna needing to be cut. Most have a screw at the base you can loosen to adjust the antenna length up or down. The biggest mobile you'll see is a 1/4 wave, or 8 feet. I had a 3/4 wave base Sigma 4. And for a while I had 2 of them cophased into a beam. ;)


Ours is one of the simple, old-fashioned 8 footers, with a loading coil at the base. We don't use it much, but it can be handy when 4 wheel driving in the bush. "Cophased", Misfit? That's not a term I've heard before. Please explain.
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Message 525927 - Posted: 3 Mar 2007, 19:37:42 UTC - in response to Message 525785.



[snip]

If a dipole is physically less than half-wavelength, it is usually "fooled" into behaving as a half-wavelength by the use of a "loading coil". Basically, this is a rheostat (variable resistor) which can be adjusted. When CB users adjust the "SWR" (Standing Wave Ratio) of their antenna, this is what they are doing.

[snip]


Generally, the loading coil is a COIL, I.E an inductor, (sometimes variable) that makes the antenna look resonant to the transmitter at a specific frequency. (lowering the SWR) For CB, this would be in the 27.200 Mhz range. (CB channel 19, the "trucker's channel" is 27.185 Mhz, but the CB band is 27.005 to 27.405.) The CB antennas available commercially are usually long, though - you CUT them to the proper length for mid band. (called "tuning the antenna")


All correct! :)
Mind you, I thought the practice of cutting the antennae to suit the transmitter had died out long ago. I suppose the added expense of a "SWR" meter puts it out of range for some people, and often the added performance just isn't needed.

[edit] The ideal length for a CB antenna with NO loading coil would be around 6.9 metres, hence most CB antenna have a built-in coil, usually at the base, but often in the centre of the "whip"

Well for a full wave that would be 11 meters. (it's in the 11 meter band!) But I've never known an antenna needing to be cut. Most have a screw at the base you can loosen to adjust the antenna length up or down. The biggest mobile you'll see is a 1/4 wave, or 8 feet. I had a 3/4 wave base Sigma 4. And for a while I had 2 of them cophased into a beam. ;)


Ours is one of the simple, old-fashioned 8 footers, with a loading coil at the base. We don't use it much, but it can be handy when 4 wheel driving in the bush. "Cophased", Misfit? That's not a term I've heard before. Please explain.

I don't know if "cophased" is an official technical term or just street talk. Cable from the radio ends in a Y connector. That is split into the two antennas. One length (I can't remember to which antenna) of the cable is longer than the other. This causes the signal to arrive at one antenna slightly behind the other. The effect of this is a unidirectional beam parallel to the antennas - as opposed to a bidirectional beam perpendicular to the antennas if the cables were the same length. The lengths of cable must be specific and determined by the radio frequency. Of course the power is split between the antennas but you more than make up for that thru the signal gain. I had a little more gain than a Moonraker 4 but without needing all that space.
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Message 526029 - Posted: 3 Mar 2007, 23:37:26 UTC - in response to Message 525785.

"Cophased", Misfit? That's not a term I've heard before. Please explain.

Phased array maybe?

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Message 526032 - Posted: 3 Mar 2007, 23:43:52 UTC - in response to Message 525346.
Last modified: 3 Mar 2007, 23:44:02 UTC



Generally, the loading coil is a COIL, I.E an inductor, (sometimes variable) that makes the antenna look resonant to the transmitter at a specific frequency. (lowering the SWR) For CB, this would be in the 27.200 Mhz range. (CB channel 19, the "trucker's channel" is 27.185 Mhz, but the CB band is 27.005 to 27.405.) The CB antennas available commercially are usually long, though - you CUT them to the proper length for mid band. (called "tuning the antenna")


All correct! :)
Mind you, I thought the practice of cutting the antennae to suit the transmitter had died out long ago. I suppose the added expense of a "SWR" meter puts it out of range for some people, and often the added performance just isn't needed.

[edit] The ideal length for a CB antenna with NO loading coil would be around 6.9 metres, hence most CB antenna have a built-in coil, usually at the base, but often in the centre of the "whip"

Ignoring the whole CB thing (which was once, and should again, be the 11 meter Amateur band and not a short-range business service).

The need to tune an antenna depends on many factors.

I'm going to ignore shortened (loaded) antennas too, at least for the moment.

The half-wave dipole is a classic antenna design, widely used. Some designs are better, most are a whole lot worse.

A half-wave dipole for the middle of the (US) A.M. broadcast band is 468 feet long.

A dipole for the 10 meter amateur band is only 16 1/2 feet long.

When you calculate the length for 2.4 GHz (WIFI) we're talking less than 3 inches.

The ratio of the wire diameter to the wire length is important too, that is a big factor for how "broad" the antenna will be -- so when you start dealing with WiFi, the conductors are huge when compared to an antenna for FM broadcast.

If the antenna is broad enough, it won't need to be tuned to a specific frequency.

There are other tricks to designing broadband antennas, and for most of the stuff that the average consumer deals with, antennas are easily made which cover from maybe 1.8 GHz to 2.7 GHz (for a WIFI antenna) or from 50 MHz to well over 500 MHz (for TV).
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Message 526033 - Posted: 3 Mar 2007, 23:44:37 UTC - in response to Message 525927.


I don't know if "cophased" is an official technical term or just street talk.

It is street talk.
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Message 526035 - Posted: 3 Mar 2007, 23:49:21 UTC - in response to Message 526029.

"Cophased", Misfit? That's not a term I've heard before. Please explain.

Phased array maybe?

CB/Trucker slang.

Take a couple of CB antennas, and bolt them on the mirrors of yer 18 wheeler. Feed them in phase, and you get a bit of a pattern, which conveniently puts a good signal out ahead, and a good signal behind you (your "back door").

This is rarely done with any real engineering to it. The antennas are about 12' apart because that's about normal on a truck.

You'd probably do better (but with less directivity) if you could center the antenna on the ground plane, but truckers didn't do that.
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Message 526039 - Posted: 3 Mar 2007, 23:56:27 UTC - in response to Message 525785.


Ours is one of the simple, old-fashioned 8 footers, with a loading coil at the base. We don't use it much, but it can be handy when 4 wheel driving in the bush. "Cophased", Misfit? That's not a term I've heard before. Please explain.

I was doing a little bit of reading, and "personal locator beacons" are now allowed in the U.S. and I'm sure are sold elsewhere.

They're a little expensive($500-ish), but not that much so.

They transmit on 406 MHz and can be located using the COSPAS/SARSAT system.

There are also receivers for this service on several geostationary satellites.

If you're lost or in trouble, you active the PLB and it transmits the serial number, which identifies you. The Search and Rescue folks will then check with family and friends (which you have registered) to see if you're around while they're waiting for the next SARSAT or COSPAS pass.

.... and the better units now have GPS, so they know exactly where you are on the very first transmission.

Kinda limits the "search" part of Search and Rescue -- and if you really get out into the sticks, you don't have to worry about calling for help and not being heard.
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Message 526041 - Posted: 4 Mar 2007, 0:05:55 UTC - in response to Message 525927.
Last modified: 4 Mar 2007, 0:26:03 UTC

Cophased is a valid term or a portion of phased array antennas (or it was IEEE)... But then radio types abused it a bit... The type of phasing of the array will develvope the Lobes for transmit and receive... The general idea is to increase the gain (across the antennas) for the same transmitted power (transmitted or received)...

I don't know if "cophased" is an official technical term or just street talk. Cable from the radio ends in a Y connector. That is split into the two antennas. One length (I can't remember to which antenna) of the cable is longer than the other. This causes the signal to arrive at one antenna slightly behind the other. The effect of this is a unidirectional beam parallel to the antennas - as opposed to a bidirectional beam perpendicular to the antennas if the cables were the same length. The lengths of cable must be specific and determined by the radio frequency. Of course the power is split between the antennas but you more than make up for that thru the signal gain. I had a little more gain than a Moonraker 4 but without needing all that space.


So in your example if both cables are the same length the expanded "beam" would be perpendiclar to the line between the antennas (additive in both send and receive exactly in phase)... or in the example of the one shorter that the other (some multiple of the wavelength for the transmitted/received frequency) the "beam" would arrive at one antenna and then the other antenna (adding to the amplitude of the signal) which would result in the signal being propagated "inline" with the antennas... This could be accomplished with multiple feedlines or a single feedline and multiple elements that are a specific distance apart and length with respect to the frequency being transmitted or received...


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Message 526070 - Posted: 4 Mar 2007, 0:39:39 UTC - in response to Message 526032.
Last modified: 4 Mar 2007, 0:48:47 UTC

The ratio of the wire diameter to the wire length is important too, that is a big factor for how "broad" the antenna will be -- so when you start dealing with WiFi, the conductors are huge when compared to an antenna for FM broadcast.

If the antenna is broad enough, it won't need to be tuned to a specific frequency.

There are other tricks to designing broadband antennas, and for most of the stuff that the average consumer deals with, antennas are easily made which cover from maybe 1.8 GHz to 2.7 GHz (for a WIFI antenna) or from 50 MHz to well over 500 MHz (for TV).


@Misfit and others, RE: "Cophased" Thanks for the explanations. I've seen such arrays, but not had to use them. I knew them by the name suggested by Grant, and should have guessed what Misfit meant.

@Ned: The broadband antenna is one I've seen a bit of. There is a good example in what is called the "Blandford antenna", designed by the Royal Signals, and named after the town/camp the School of Signals is located at.

The thing is a short, squat affair which looks like a bundle of overweight wire coathangers have been stuck together, bent into rhomboid shapes and stuck in the ground. The trick is the thickness of the metal it's made of.

The personal locator devices are called EPIRBs here, (No totally sure of the full name, but it doesn't matter) and are the same as the ones used by boats in distress. Most situations, I think, would require less dire actions when "bush-bashing". CB and UHF comms are useful to the extent that info re: conditions in a given area can be gained from other users. That sort of thing.
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Message 526147 - Posted: 4 Mar 2007, 2:18:51 UTC - in response to Message 526070.


@Ned: The broadband antenna is one I've seen a bit of. There is a good example in what is called the "Blandford antenna", designed by the Royal Signals, and named after the town/camp the School of Signals is located at.

The thing is a short, squat affair which looks like a bundle of overweight wire coathangers have been stuck together, bent into rhomboid shapes and stuck in the ground. The trick is the thickness of the metal it's made of.

The personal locator devices are called EPIRBs here, (No totally sure of the full name, but it doesn't matter) and are the same as the ones used by boats in distress. Most situations, I think, would require less dire actions when "bush-bashing". CB and UHF comms are useful to the extent that info re: conditions in a given area can be gained from other users. That sort of thing.

I can think of at least two more examples of pretty broadband antennas.

The first is a Log-Periodic Dipole Array. Most TV antennas are based on this design -- lots of different length bits, and some being too long are "inductive" and some being too short are "capacitive" and the inductance and capacitance cancel.

The second is called a discone -- which surprisingly is composed of a disk at the top, and a cone at the bottom -- and these are usually approximated by wires instead of solid metal.

At least in the US, an EPIRB is legal only on a boat, while an ELT (Emergency Landing Transmitter) is on an airplane. You can have a personal locator beacon in your backpack, or in the trunk of your car.
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Message 526250 - Posted: 4 Mar 2007, 11:42:41 UTC

havent heard from eric for a while have you fixed it yet dude or decided to sell it and buy a new one

All the best from the newbie from down under great work

Troy

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Message 527022 - Posted: 5 Mar 2007, 23:23:29 UTC - in response to Message 526147.


The first is a Log-Periodic Dipole Array. Most TV antennas are based on this design -- lots of different length bits, and some being too long are "inductive" and some being too short are "capacitive" and the inductance and capacitance cancel.


Is that a Yaggi array?
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Message 527089 - Posted: 6 Mar 2007, 3:13:32 UTC - in response to Message 527022.
Last modified: 6 Mar 2007, 3:14:35 UTC


The first is a Log-Periodic Dipole Array. Most TV antennas are based on this design -- lots of different length bits, and some being too long are "inductive" and some being too short are "capacitive" and the inductance and capacitance cancel.


Is that a Yaggi array?

The driven element on a Yagi (Yagi-Uda) antenna is a dipole. The other elements are not driven.

On a log-periodic, all of the elements are driven.

(p.s. alot of these antennas can be used reasonably well as feedpoint antennas for a dish)
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Message 527181 - Posted: 6 Mar 2007, 6:27:46 UTC - in response to Message 527089.
Last modified: 6 Mar 2007, 7:24:48 UTC


The first is a Log-Periodic Dipole Array. Most TV antennas are based on this design -- lots of different length bits, and some being too long are "inductive" and some being too short are "capacitive" and the inductance and capacitance cancel.


Is that a Yaggi array?

The driven element on a Yagi (Yagi-Uda) antenna is a dipole. The other elements are not driven.

On a log-periodic, all of the elements are driven.

(p.s. alot of these antennas can be used reasonably well as feedpoint antennas for a dish)


The basic Yagi (pronounced: yargee) array has three elements:

1) The dipole, which is connected to the co-ax, and is thus "driven" as Ned said.

2) The reflector, which is longer than the dipole, and collects radio waves, working rather like a dish does on a parabolic antenna, by reflecting the received waves back to the dipole.

3) The concentrator, which is shorter than the dipole, and has the effect of producing a more directional beam, thus increasing gain, or sensitivity.

Often, there are several concentrators, and sometimes multiple reflectors are employed. I have never seen a Yagi antenna with multiple dipoles.

The result is a directional antenna, which should be pointed in the direction of the transmitter (or receiver, if the antenna is used to transmit). Again, as Ned says, they are the most popular form of TV antenna.

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